The periodic formation of a domain inversion structure for forcibly inverting the polarization of ferroelectrics allows the implementation of optical frequency modulators utilizing surface acoustic waves, optical wavelength conversion devices utilizing the polarization inversion of nonlinear polarization, and the like. In particular, if the nonlinear polarization of nonlinear optical materials can be inverted periodically, high-efficiency wavelength conversion devices can be made; when light such as solid lasers is converted by using such devices, it will be possible to make small and lightweight short-wavelength light sources applied for such fields as printing, optical information processing, and optical application measurement control.
As a method of forming a periodic domain inversion structure in a ferroelectric nonlinear optical material, so-called voltage application method is known. In this method, an interdigitated comb-shaped electrode is formed on one main face of a ferroelectric single crystal substrate, a uniform electrode is formed on the other main face, and a pulse voltage is applied across both the electrodes. Such a method is described in Japanese Publication Nos. H08-220578A, 2005-70195A and 2005-70194A.
In order to generate second harmonics from a nonlinear optical material such as a lithium niobate single crystal, there is a need to make periodic domain inversion within the single crystal. Then, the periodic domain inversion structure is formed in a ferroelectric single crystal substrate, following which a ridge channel optical waveguide is formed in a surface of the substrate by machining, laser machining, or the like. At that time, by providing the periodic domain inversion structure within the ridge optical waveguide, fundamental waves incident on the optical waveguide are modulated to higher harmonic waves.